1) Field of the Invention
The present invention relates to a method and an apparatus for recording signals by irradiating a bundle of high-density energy on an optical information recording medium.
2) Description of the Prior Art
The technique of recording and reproducing information signals utilizing a laser beam has been known and is currently applied to document files and data files extensively. Moreover, examples of research development of a rewritable recording system having an erasing function are also being reported.
One of the above methods entails a phase-change typed optical recording medium utilizing a reversible change of state either between amorphous and crystalline or between crystalline and crystalline. A recording layer for the optical recording medium is characterized in that an amorphous state or a crystalline state is formed and these states are reversibly changed according to the reached temperature and the cooling condition in the course of irradiating the laser beam. The amorphous state is different from the crystalline state in the complex index of refraction consisting of the index of refraction n and the extinction coefficient k, and the signals are recorded utilizing the difference of optical transmission or reflection.
To realize the aforementioned operation, the method of irradiating a laser beam modulated between a recording power level Pw and an erasing power level Pe .(Pw&gt;Pe) on the recording medium is proposed in, for example, the Japanese Journal of Applied Physics, Vol. 26 (1987), Suppl. 26-4, p. 61-66. When the laser beam modulated in response to information signals is irradiated on the recording layer, the portion where the recording power Pw is irradiated is turned to be in the amorphous state and the portion where the erasing power Pe is irradiated is turned to be in the crystalline state regardless of the state thereof before the laser beam irradiation. As a result, the recording and the erasing can be simultaneously be carried out by one spot, and thus overwriting is made possible. Here, while the change of state between amorphous and crystalline has been described above, the change of state between crystalline and crystalline results in the same by the correspondence of the above amorphous state to a second crystalline state.
As described above, the laser beam modulated between two power levels is irradiated to perform information recording on the optical disk. However, if the signals are repeatedly recorded on the same track, the recording medium is gradually degraded according to the thermal history, and as a result, the amplitudes of reproduced signals are apt to be decreased. Especially, there is a problem that in when a signal of a constant pattern is repeatedly recorded on a same position on the recording medium and thereafter signals of different patterns are recorded, the amplitude of reproduced signals is deviated and read errors of data (bit error) occur in the process of demodulating the signals. The number of repetition recordings over which the above bit error is substantially caused becomes smaller in comparison with the case of recording different signals every time.
On the recording medium on which the signals of the same pattern are repeatedly recorded, there are a portion on which only the intense laser beam (of the recording power level) is condensed and a portion on which only the feeble laser beam (of the erasing power level) is irradiated in the aforementioned phenomenon. For this reason, adjacent portions have been subjected to different times of repetition recordings. When a signal of another pattern is recorded on such portions and reproduced, the amplitude of the reproduced signal varies depending on the laser beams that have irradiated the recorded portion in the past. Namely, since the thermal deterioration is significant in the portion where the intense laser beam has always been condensed and irradiated, the reproduced signal amplitude is decreased. On the other hand, since the thermal deterioration is slight in the portion where only the feeble laser beam has been irradiated, the reproduced signal amplitude is as large as in the initial condition. When the aforementioned difference in amplitude has occurred, peak shift and a lack of signals occur in demodulating the data signals, thereby causing bit errors. It is difficult to consider that the same data signals are repeatedly recorded in the actual recording. However, even if the data value itself is changed in recording the data signals, the pattern of a frame signal giving indications between data, a clock signal portion or the like is the same. Therefore, when the information signals are repeatedly recorded more frequently, the errors would be caused for the above reason.